[llvm] r349944 - [RuntimeUnrolling] NFC: Add TODO and comments in connectProlog

[Anna Thomas via llvm-commits]

Author: annat
Date: Fri Dec 21 11:45:05 2018
New Revision: 349944

URL: http://llvm.org/viewvc/llvm-project?rev=349944&view=rev
Log:
[RuntimeUnrolling] NFC: Add TODO and comments in connectProlog

Currently, runtime unrolling does not support loops where multiple
exiting blocks exit to the latchExit. Added TODO and other code
clarifications for ConnectProlog code.

Modified:
    llvm/trunk/lib/Transforms/Utils/LoopUnrollRuntime.cpp

Modified: llvm/trunk/lib/Transforms/Utils/LoopUnrollRuntime.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Utils/LoopUnrollRuntime.cpp?rev=349944&r1=349943&r2=349944&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Utils/LoopUnrollRuntime.cpp (original)
+++ llvm/trunk/lib/Transforms/Utils/LoopUnrollRuntime.cpp Fri Dec 21 11:45:05 2018
@@ -70,6 +70,17 @@ static void ConnectProlog(Loop *L, Value
                           BasicBlock *PreHeader, BasicBlock *NewPreHeader,
                           ValueToValueMapTy &VMap, DominatorTree *DT,
                           LoopInfo *LI, bool PreserveLCSSA) {
+  // Loop structure should be the following:
+  // Preheader
+  //  PrologPreHeader
+  //  ...
+  //  PrologLatch
+  //  PrologExit
+  //   NewPreheader
+  //    Header
+  //    ...
+  //    Latch
+  //      LatchExit
   BasicBlock *Latch = L->getLoopLatch();
   assert(Latch && "Loop must have a latch");
   BasicBlock *PrologLatch = cast<BasicBlock>(VMap[Latch]);
@@ -83,14 +94,21 @@ static void ConnectProlog(Loop *L, Value
     for (PHINode &PN : Succ->phis()) {
       // Add a new PHI node to the prolog end block and add the
       // appropriate incoming values.
+      // TODO: This code assumes that the PrologExit (or the LatchExit block for
+      // prolog loop) contains only one predecessor from the loop, i.e. the
+      // PrologLatch. When supporting multiple-exiting block loops, we can have
+      // two or more blocks that have the LatchExit as the target in the
+      // original loop.
       PHINode *NewPN = PHINode::Create(PN.getType(), 2, PN.getName() + ".unr",
                                        PrologExit->getFirstNonPHI());
       // Adding a value to the new PHI node from the original loop preheader.
       // This is the value that skips all the prolog code.
       if (L->contains(&PN)) {
+        // Succ is loop header.
         NewPN->addIncoming(PN.getIncomingValueForBlock(NewPreHeader),
                            PreHeader);
       } else {
+        // Succ is LatchExit
         NewPN->addIncoming(UndefValue::get(PN.getType()), PreHeader);
       }